1. Field of the Invention
The present invention relates to a solder-packaging method for an electronic device, and an electronic module manufactured by using the method.
2. Description of the Related Art
Conventionally, Sn--Pb based solder is employed for joining an electronic device by using solder. In particular, in order to reduce the temperature load of the parts, solder having composition near 63Sn--37Pb and a melting point of about 183.degree. C. (hereinafter merely called eutectic solder) has been widely used. The terminals (leads) or the electrodes of the electronic parts etc. are joined to the Cu electrode pads on the circuit board of a printed circuit etc by using such solder and heating to about 220.degree. C.
On the other hand, in recent years, in a view point of the environmental pollution, such a tendency of restricting the usage of lead (Pb) contained in solder for an electronic device has been active. Thus, solder not containing Pb, that is, so-called "Pb-free solder" has been developed and the application thereof has been popularly investigated.
The Pb-free solder alloy having been developed is formed in a manner that a small amount of third and fourth elements such as Cu, Bi, In etc are added to the Sn-type eutectic solder based alloy including Sn--58Bi (melting point; 139.degree. C.), Sn--3.5Ag (melting point: 221.degree. C.), Sn--9Zn (melting point: 197.degree. C.) etc., in order to adjust the melting point and improve the mechanical property thereof. However, such Pb-free solder alloys have both a good point and a bad point in the joining reliability (wettability, heat-resistant fatigue etc.), working temperature (melting point), stability of supply, cost etc., so that it is difficult under the existing circumstances to employ a single Pb-free solder in place of the current eutectic solder so as to satisfy the aforesaid properties for all of the flow soldering, reflow soldering and hand soldering.
In particular, in the reflow soldering in which a large number of parts mounted on a board can not be heated partially or locally but are soldered simultaneously, since it is required to heat all the soldering portions to a predetermined temperature, there may be portions which temperature increases above the required value. Thus, there have been various restrictions in the heat-resistant temperature of the parts and the material of the solder to be used. In general, solder alloy which does not remelt at the temperature equal to or less than 150.degree. C. is employed in view of securing the reliability with respect to the heat generated upon operation of a semiconductor element and the thermal history in the assembling process thereafter. In contrast, in view of the temperature variance at the time of mounting a large number of parts, and in order to prevent the degradation of the quality of the parts with poor heat-resistant property such as a chemical capacitor and the polymer material such as a print circuit board and to prevent the degradation of electric elements etc., it is desired to set the temperature load applied to the parts joined by the eutectic solder at about 220.degree. C. or lower than that.
In view of the aforesaid matters, at present, Sn--9Zn based alloy having a melting point of about 187-197.degree. C. at most has been developed as the Pb-free solder in a view point of the melting point of the alloy. Such an alloy is quite likely oxidized since Zn is very active, so that wettability can not be higher reliability under the current soldering process even if various ideas such as the heating using strong flux or within inactive atmosphere are employed. Even if higher wettability can be obtained, when the electronic device having been subjected to the solder joining is used under the usual circumstances, Zn diffuses within the solder joining portion and appears on the surface of the joining portion to form a Zn-enriched layer. When such solder is coated on a terminal thereby to cure the mold resin, a Zn-enriched layer is formed on the surface of the package terminal through the thermal history of the solder. In this case, there arises such problems that higher wettability can not be obtained due to the oxidization of the Zn-enriched layer and the stress corrosion sensibility is enhanced, so that it is difficult at the present stage to employ other solder in place of the aforesaid solder.
On the other hand, there is another type of soldering material which is formed by adding Bi, In having melting-point lowering effect to Sn--3.5Ag such soldering material has such a property that solidus curve temperature decreases but liquidus curve little decreases. Thus, in a case of joining by using such soldering material under the current soldering process, the joining process is performed in an area where both solidus and liquidus exist. That is, the joining is performed by the liquidus portion while remaining the solidus portion. Thus, in this case, a coefficient of viscosity is larger and the wettability is lower as compared with the case where the soldering is performed in a state that the entirety of the solder is in a liquidus state, so that the defective joining such as a void etc. is likely generated. When a large amount of the aforesaid elements are added, there arises a problem that the cost of the soldering material becomes expensive and the reliability of the joining process is degraded.
As described above, such a solder joining technology is desired that employs, in place of the current eutectic solder, Pb solder which can join by using the existing device and has high joining reliability.
Further, there is a module wherein Pb-free material is employed only in the soldering material and the plating process using the material containing Pb is performed for the metalization on the parts side. Such a module is not an electronic module fabricated by the Pb-free solder joining process. Thus, an electronic module fabricated by the Pb-free solder joining process in the true meaning is desired.
In the aforesaid soldering technology for an electronic device etc., it is desirable that a large number of parts can be soldered simultaneously. Further, it is desirable that the joining temperature at the soldering process is low in view of the joining reliability and it is also necessary the solder does not remelt by the heat generated upon operation of electric elements.
However, the aforesaid prior art has the following problems.
Each of the Sn--Ag type alloy and the alloy formed by adding Bi, In etc. thereto has a melting point of 221.degree. C. which is higher than the melting point of the current Sn--Pb eutectic solder by 40.degree. C., so that it is impossible to perform the joining process under the current joining temperature at the soldering process. Thus, in a case of performing the soldering by using such alloy as the soldering material, the soldering is required to be performed after removing parts with a low heat-resistant temperature and the parts thus removed are joined thereafter, so that the soldering cost becomes expensive. Further, since the peak temperature of the heating process etc. is made high, polymer material such as the electric elements, resin etc. is likely deteriorated heavily and the parts are likely subjected to oxidization etc. heavily, whereby it is feared that defective soldering may be caused. Furthermore, in the area where both solidus and liquidus exist, although the solder is in a semi-melting state, all the solder is not in the liquidus state, so that it is feared that high wettability may not be obtained. In particular, it is feared that the defective joining such as a void etc. may be generated for the soldering portion subjected to the surface processing using Ni etc. having poor wettability.
Each of the Sn--Bi type alloy and the alloy formed by adding some kinds of elements thereto has a melting point of 139.degree. C. which is lower than the melting point of the eutectic solder by 50.degree. C., so that such alloy may remelt by the heat generated upon the operation of an electric element thereby to degrade the joining reliability. Further, the joining reliability may also be degraded due to the fragility of Bi much contained in the alloy.
Each of the Sn--Zn type alloy and the alloy formed by adding small amount of Bi, In etc. thereto having a melting point closest to that of the eutectic solder is oxidized heavily under a particular circumference such as the heating using strong flux or within inactive atmosphere, so that it is difficult to obtain high wettability and so it is feared that the joining reliability is degraded.
As described above, it is impossible at present to employ alloy made of single composition in place of the eutectic solder.
In order to solve the aforesaid problems, for example, JP-A-10-41621 discloses, as a method of improving the solder joining reliability of Sn--Bi type alloy, a method of supplying Sn--Bi alloy to a part side, supplying an Ag additive film to a circuit board side and diffusing Ag into a Sn--Bi solder layer upon soldering. However, since the Sn--Bi is heated to a temperature higher than the melting point thereof and joined, the constituent metal or metalized material diffuses within the solder layer from the part side in a short time thereby to accelerate the growth of the intermetallic compound. As a result, it is feared that the joining reliability is degraded due to the growth of the intermetallic compound.
Further, in view of the fact that the composition of the Ag additive layer is not disclosed clearly and the melting point of Ag is 962.degree. C., it is feared that the temperature etc. for obtaining good joining state with the Sn--Bi alloy having a melting point around 139.degree. C. varies largely, whereby such solder is disadvantageously insufficient to be used generally in place of the existing eutectic solder. Furthermore, since the main alloy is formed by the Sn--Bi alloy, the remelting temperature of the alloy does not increase largely even if a small amount of Ag is diffused therein, so that it is feared that such alloy may remelt around 139.degree. C. Thus, in order to diffuse a large amount of Ag, it is required to make the heating temperature higher or to make the heating time longer. In this case, it is feared that the cost of such solder is raised due to the increase of Ag.
On the other hand, as a method of forming a mixed integrated circuit module, for example, JP-A-55-14941 discloses such a method of supplying high melting point solder to some of IC chips, supplying low melting point solder to a board side in opposite thereto and to a non-IC chip, heating at such a temperature range that the lower melting point solder melts but the higher melting point solder does not melt, thereby to perform melting joining in a collective reflow manner without degrading IC elements. This method is a technique particularly being conscious of Sn--Pb alloy which melting temperature can be designed easily. In the Pb-free solder, since some of non-IC chips is joined by lower melting point alloy, for example, Sn--58Bi, it is feared as described above that defective remelting due to the heat generated upon operation of the element may be caused and the joining reliability may be degraded due to the excessive Bi as compared with the existing alloy.